Anisotropy of band gap absorption in TlGaSe2 semiconductor by ferroelectric phase transformation

Abstract

The depth-resolved free-carrier absorption and the photo-acoustic response are used to examine the band-gap absorption in 2D-TlGaSe2 layered semiconductor after its transformation into the ferroelectric F-phase below 107 K. The absorption exhibits unusual behavior with a biaxial character in respect to the light polarization on the layer plane. A spectral analysis shows that the anisotropy is associated to the lowest Γ-direct optical transition. The Γ-absorption and the localized exciton at 2.11 eV are dipole-prohibited or partially allowed in two nearly perpendicular polarization directions. The shift of anisotropy axis in respect to crystallographic a- and b-directions demonstrates the non-equivalent zigzag rearrangement of the interlayer connecting Tl+ ions, which is responsible for occurrence of the F-phase.

Carrier dynamics in highly excited TlInS2: evidence of 2D electron–hole charge separation at parallel layers

“…This is a kind of hybridization of electron lone pairs. [8,9,11] Such excess carrier separation does not exist in isotropic semiconductor crystals. The crystal phase transition by the use of light can be simply understood as a minimization of the potential energy by a sufficiently strong separation of e-h pairs.…”

“…On cooling the TlInS 2 crystal loses inversion center undergoing two sequential transitions via incommensurate I-phase into ferroelectric F-phases. Tl + ion deviation and grouping occurs in the interlayer [8,9] and the I-phase is accompanied by strong resonance of complex dielectric function. [10] The lattice unit quadrupling within I-phase occurs around 200 K. [5] Weakness of interlayer bonds results from the sp-hybridization of the Tl electrons wave function where a strong coupling among the spins of Tl and In metal ions on neighboring layers is observed.…”

“…The PSFs can make invariant of a to b axis, e.g. this shifting can lead to tilting monolithic structure from [110] to [1][2][3][4][5][6][7][8][9][10]. Even though the PSFs cannot be associated with broken bonds, the PSFs edges may give rise to different stresses and band gap states.…”

Photo-acoustic response and optical features of 2D-TlGaSe2 and GaAs semiconductors

“…The unit cell contains 16 atoms positioned on two adjacent layers rotated by 90° angle in (100) plain. The Tl atoms lie in perpendicular trigonal cavities between layers and provide a weak interlayer covalent bonding [1,[3][4][5][6][7]. Due to the sufficient length of these bonds, there is a predisposition for electrical anisotropy.…”

“…In recent years, TlGaSe 2 and TlInS 2 compounds have gained significant interest because of discovering their unique, unexpected electro-optical properties. In conjunction with previously known sequential phase transitions occurring at low temperatures, extensively studied during last decades [1,3,4], recent investigation indicated electro-optical metastable transformations if samples were thermo-cycled or kept for a certain period in the external electric fields (see [5] and references therein). Our previous works have shown that Tl-GaSe 2 as well as the equivalent TlInS 2 compound obey phenomena of temporal or permanent photo-darkening (giant Stark effect) after a prolonged intense laser pulse irradiation [6,7].…”